The invention relates generally to integrated circuit fabrication and semiconductor devices and, in particular, to methods for designing and fabricating a current mirror.
A current mirror may be used as a current source in a wide range of electronic circuits, such as power amplifiers. In a common configuration, a reference current source supplied a constant reference current to a reference bipolar junction transistor, which passes the reference current at the same level as the reference current source to a mirrored current source. The mirrored current source includes one or more additional mirrored bipolar junction transistors that also pass current from an external power supply, but with reference to the reference current from the reference transistor. Consequently, the reference current source is used to control the output of the mirrored current source.
A bipolar junction transistor is a three-terminal electronic device that includes an emitter, an intrinsic base, and a collector defined by regions of different semiconductor materials. In the device structure, the intrinsic base is situated between the emitter and collector. An NPN bipolar junction transistor may include n-type semiconductor material regions constituting the emitter and collector, and a region of p-type semiconductor material constituting the intrinsic base. A PNP bipolar junction transistor includes p-type semiconductor material regions constituting the emitter and collector, and a region of n-type semiconductor material constituting the intrinsic base. In operation, the base-emitter junction is forward biased, the base-collector junction is reverse biased, and the collector-emitter current may be controlled by the base-emitter voltage.
A heterojunction bipolar transistor is a variant of a bipolar junction transistor in which at least two of the collector, emitter, and intrinsic base are constituted by semiconductor materials with different energy bandgaps, which creates heterojunctions. For example, the collector and/or emitter of a heterojunction bipolar transistor may be constituted by silicon, and the base of a heterojunction bipolar transistor may be constituted by silicon germanium (SiGe), which is characterized by a narrower band gap than silicon.
Improved methods are needed for designing and fabricating a current mirror.